Complementary Metal Oxide Semiconductor (CMOS) image sensors typically include multiple pixels that are arranged in rows and columns to form a pixel array. A column output circuit is connected to each column of pixels in the array to read out the signals from the pixels in the column. The output signals from a row of pixels are read out of the pixel array one row at a time (in parallel) by the column output circuits.
The column output circuits can include both analog and digital circuits, including capacitors, switches, multiplexers, transistors, and amplifiers. The quality of an image captured by a CMOS image sensor can be reduced due to differences or mismatches between the offsets and the gains of the circuitry in the column output circuits. These mismatches produced column fixed pattern noise artifacts or defects in the images. Column fixed pattern noise is more visible and objectionable to an observer due to the columnar or striped structure of the column fixed pattern noise.
When an image is captured, a gain is often applied to the output signals of the pixels. This is especially true when the image is captured in low light conditions, where the gain is used to make the image brighter. Unfortunately, column fixed pattern noise is also amplified by the gain, making the column fixed pattern noise artifacts even more noticeable in an image. For example, a twofold increase in the gain can result in a doubling of the column fixed pattern noise.